Because an organic EL device is a self-luminescent device, it is luminous, excellent in visibility, and capable of giving clear display, as compared with a liquid crystal device. Therefore, active investigations have been made.
C. W. Tang et al. of Eastman Kodak Company developed a two-layer type laminated structure element in 1987, and this enabled an organic EL device using an organic substance to be put into practical use. They laminated an electron transporting fluorescent substance and a hole transporting organic substance, and injected both charges in a layer of the fluorescent substance to make the layer emit, thereby making it possible to attain high luminance of 1,000 cd/m2 or more at a voltage of 10V or lower (for example, see Patent Document 1 and Patent Document 2).    Patent Document 1: JP-A-8-48656    Patent Document 2: Japanese Patent No. 3194657
Up to the present, many improvements have been made to put an organic EL device into practical use. High efficiency and durability are achieved by an electroluminescence device in which the role of two layers is further finely divided, and an anode, a hole injecting layer, hole transporting layer, an emission layer, an electron transporting layer, an electron injecting layer and a cathode are provided successively on a substrate (for example, see Non-Patent Document 1).
Non-Patent Document 1: The Japan Society of Applied Physics, 9th Lecture, Extended Abstract, pages 55-61 (2001)
Further, for the purpose of further improvement of emission efficiency, utilization of a triplet excitation is attempted, and utilization of a phosphorescent substance is investigated (for example, see Non-Patent Document 2).
Non-Patent Document 2: The Japan Society of Applied Physics, 9th Lecture, Extended Abstract, pages 23-31 (2001)
The emission layer may be prepared by doping an electron transporting compound generally called a host material with a fluorescent substance or a phosphorescent substance. As described in the above extended abstracts, selection of an organic material in an organic EL device greatly affects various characteristics such as efficiency and durability of the device, etc.
In an organic EL device, emission is obtained by recombination of electric charges injected from both electrodes in an emission layer. However, because hole mobility is higher than electron mobility, efficiency reduction due to that part of holes passes through the emission layer becomes problematic. For this reason, an electron transporting material having high electron mobility is demanded.
Tris(8-hydroxyquinoline)aluminum (hereinafter referred to as Alq) that is a representative emission material is generally used as an electron transporting material, but it is said that the electron mobility therewith is slow. For this reason, as a material having high mobility, for example, 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (hereinafter referred to as PBD) was proposed (for example, see Non-Patent Document 3).    Non-Patent Document 3: Jpn. J. Appl. Phys., 27, L269 (1988)
However, it is pointed out that PBD is poor in stability in a thin film state such that it is liable to cause crystallization, and various oxadiazole derivatives have been proposed (for example, see Patent Documents 3 to 5).    Patent Document 3: Japanese Patent No. 2721442    Patent Document 4: Japanese Patent No. 3316236    Patent Document 5: Japanese Patent No. 3486994
In those electron transporting materials, although stability was improved as compared with PBD, it cannot be considered as sufficient, and from the standpoint of balance with a hole mobility, the electron mobility was still insufficient. For this reason, there were many cases that Alq having good stability is used as the electron transporting material, but satisfactory device characteristics were not obtained.
Further, as measures of preventing part of holes from passing through an emission layer and improving probability of recombination of electric charges in the emission layer, there is a method of inserting a hole blocking layer. As a hole blocking material, triazole derivatives (for example, see Patent Document 6), bathocuproine (hereinafter referred to as BCP), a mixed rigand complex of aluminum (BAlq) (for example, see Non-Patent Document 2) and the like are hitherto proposed.    Patent Document 6: Japanese Patent No. 2734341
However, in either material, film stability is insufficient, or function to block holes is insufficient. A hole blocking material generally used at present is BCP. However, BCP is not considered as a sufficiently stable material, so that it is not considered as sufficiently functioning as a hole blocking layer. Thus, satisfactory device characteristics were not obtained.
To improve device characteristics of an organic EL device, an organic compound having excellent electron injecting and transporting performances and hole blocking ability, and high stability in a thin film state is demanded.